The present invention relates in general to the operation and control of internal combustion engines. More particularly, the invention provides an improved air-assisted fuel injection system and a fuel injector for use in that system.
In recent years, both federal and state governments have enacted laws and established regulations which impose even greater restrictions on the performance of motor vehicles in the areas of exhaust gas emission and fuel economy. Great strides have been made in developing control systems and components for internal combustion engines in an effort to meet these requirements. Most notable have been the advances made in fuel injection systems and in fuel injectors themselves.
In developing new fuel injection systems, emphasis has been placed on more precise fuel metering to each cylinder of the internal combustion engine to not only achieve a greater fuel economy, but also to improve the efficiency of combustion leading to reduced components of NO.sub.x, HC, CO and CO.sub.2 in the exhaust emission. Further advances have also been made in development of fuel injectors capable of producing a fuel spray of reduced particle size, which increases the combustion efficiency and enhances cold start performance of the engine.
One type of fuel injection system which is attracting attention once again is the air-assisted system in which air under pressure is supplied to the fuel injectors to mix with the liquid fuel for purposes of achieving higher levels of atomization. One of the earlier attempts at achieving greater atomization of fuel is described in the Leflaive U.S. Pat. No. 1,027,054, issued May 21, 1912, which discloses a needle valve injector in which compressed air and liquid fuel are mixed in an annular chamber and the resulting air-fuel mixture is then injected in response to the pressure of the injecting air through a plurality of tangentially arranged slots at an exit opening of the injector. Metering of the air-fuel mixture is effected by displacing the needle valve with respect to the exit opening.
Later efforts to improve air-assist fuel injection to achieve smaller particle size involved a swirling or rotating of the air and/or fuel streams to more efficiently break up the fuel particles being injected. As an example, the Boltz et al U.S. Pat. No. 3,872,639 provides a fuel injection apparatus in which fuel is injected through an orifice to produce a spray of relatively large particles in a cylindrical first chamber which communicates with a coaxial frustro-conical second chamber into which an air stream is introduced in a tangential direction so as to produce an air swirl into which the fuel spray from the cylindrical first chamber mixes. The resulting swirling air-fuel mixture is discharged through an exit opening in the second chamber under the pressure of injecting air. Thus, Boltz et al provides for swirling of the air before introducing the spray of fuel therein to further reduce the fuel particle size in the injected spray.
Air-assist fuel injectors are also disclosed in the Eckert U.S. Pat. Nos. 3,656,693, issued Apr. 18, 1972, the Schweizer U.S. Pat. No. 4,351,304, issued Sep. 28, 1982, and the Knapp et al U.S. Pat. No. 4,361,126, issued Nov. 30, 1982, and all assigned to Robert Bosch GmbH. In Eckert, fuel is first swirled and then mixed with radially directed air streams to produce an air-fuel mixture which is injected through a nozzle opening to form a fuel spray. Thus, in contrast to Boltz et al, in the Eckert injector, the fuel is swirled, but not the air, prior to mixture. In the later Schweizer and Knapp et al patents, air-assist injectors are disclosed in which neither the fuel nor the air is swirled prior to mixture.
The Simmons et al U.S. Pat. No. 3,474,970, issued Oct. 28, 1969, discloses an air-assist nozzle designed primarily for use in aircraft gas turbines in which a concentric conical air stream and conical fuel sheet are merged at the exit orifice of a nozzle to provide good atomization of fuels having a wide range of viscosities under various temperature and air pressure conditions. In the Matsuoka et al U.S. Pat. No. 4,434,766, issued May 6, 1984, an air-assist fuel injector is provided with tangentially arranged air intake ports of various size and configuration for producing a swirling air flow into which a fuel spray is injected.
In providing a source of pressurized air to an air-assist fuel injector, it has been common to tap air from a point in the intake manifold, typically downstream of the air-flow sensor and upstream of the throttle valve. However, to ensure a sufficient supply of air it has been proposed additionally to use an air pump to enhance the air pressure supplied to the injector, as discussed in the Igashira et al U.S. Pat. No. 4,465,060, issued Aug. 14, 1984.
Finally, in the Yamauchi et al U.S. Pat. No. 4,995,367 issued Feb. 26, 1991, there is disclosed a system for controlling fuel injection and ignition timing in an internal combustion engine operating on a mixture of gasoline and methanol in which an amount of compressed air injected with the fuel mixture through the fuel injector is controlled in accordance with the mixture ratio of the fuel mixture.
Although the above-mentioned patents disclose various types of air-assist fuel injectors which provide ways to achieve improved levels of fuel atomization, it has been difficult to produce particle sizes much below 100 .mu.m. However, to achieve the levels of fuel economy and emission control embodied in low emission vehicle (LEV) and ultra low emission vehicle (ULEV) regulations now proposed for the industry, it is important to achieve a level of atomization in fuel injection which provides a particle size of approximately 40 .mu.m, especially for improved cold start performance.
Even more importantly, in spite of the advances in fuel injector technology for air assist of fuel injection, little attention has been given to controlling the air assist in accordance with engine operating conditions to maximize the efficiency of engine operation under such conditions as cold start, partial load (cruising), partial acceleration, partial deceleration, normal acceleration and deceleration, idle running and the like.